A switching power supply device, which is a power converter, is required to be compact in size. For size reduction, it is proposed to reduce size of, for example, passive elements used in the switching power supply device. For size reduction of the passive elements, it is proposed to increase a switching frequency. The increase in switching frequency causes an increase in switching loss. As a result, a power conversion efficiency is lowered.
In JP-A-2012-70467, for example, four semiconductor switches and one snubber capacitor is additionally provided in a switching power supply device, which is formed of a step-up and -down type chopper (bidirectional chopper or two-output chopper), so that the switching elements forming the bidirectional chopper are turned off with less loss.
However, according to the configuration of the above-referred patent document, the number of the semiconductor switches, which are main elements determining circuit cost, to be added is as many as four. Further, for driving two of them, two floating power sources, the reference potentials of which are different from the ground potential, need be added. In addition, large currents as large as a current of a reactor flow momentarily in those added switches when the switching elements turn off. For this reason, it is necessary to set a rated current of the added semiconductor switches to be as large as that of the main switching elements. This will add cost and increases a mounting area of a circuit.